By now, drivers should know that auto mpg claims can be exaggerated. If you buy a car that’s listed as getting 47 mpg, you might be satisfied with, say, an average of 45 mpg. But 37 mpg?

In the grand scheme, 37 mpg is pretty decent—except when you purchase a vehicle that promises mileage that’s 10 mpg better. Two Ford hybrids, the Fusion Hybrid SE and the C-Max SE, boast overall ratings of 47 mpg on their windows in new-car dealerships. Yet a new Consumer Reports study indicates that these mileage claims are way overblown.

This is hardly the first test showing that automaker mpg figures are questionable, but there are a couple of things that are particularly surprising about this new study. First is the extent to which the MPG ratings appear to be inflated. In tests, CR found that the Ford Fusion Hybrid SE got 39 mpg overall, which is 8 mpg less than its official EPA rating of 47 mpg. The Ford C-Max SE also boasts an EPA rating of 47 mpg, but the vehicle averaged just 37 mpg in Consumer Reports’ real-life, on-the-road tests.

The second surprising aspect of CR’s announcement is just how common it is for “official” EPA ratings to be higher than the results found in independent testing. This seems to be the case especially when it comes to hybrid vehicles. In previous CR tests, models such as the Toyota Prius, Honda Civic Hybrid, Hyundai Sonata Hybrid, and Honda Insight EX were all shown to get worse gas mileage than what EPA ratings promised. As Consumer Reports points out, it’s the automakers themselves that come up with their own EPA ratings:

It’s worth noting that automakers mostly self-certify their cars. Then, the EPA spot-checks about 15 percent of them with its own tests in a lab.

This may seem like a student grading his own essay, or a food manufacturer rating its own products for nutrition. The idea, though, is that the threat of the EPA’s independent testing keeps automakers mostly honest. Very recently, the system exposed the fact that many Kia and Hyundai models had inflated mpg ratings, which were only brought to light after owners complained and the EPA investigated. The affected models had to have their mpg ratings changed, and folks who own the vehicles with inflated mileage figures are getting small refunds based on how much they drive.

Consumer Reports has passed along its Ford testing data to the EPA, and seeing as the gap in mpg findings is so substantial, it would seem necessary for the EPA to take action with its own tests.

But how could such a sizeable gap come about in the first place? Did Ford just make up the numbers with the hope that they would help sell cars, and that nobody would notice? Not at all, a Ford statement released in response to the study says:

“Early C-MAX Hybrid and Fusion Hybrid customers praise the vehicles and report a range of fuel economy figures, including some reports above 47 mpg. This reinforces the fact that driving styles, driving conditions, and other factors can cause mileage to vary.”

Presumably, because mpg ratings are supposed to reflect average, normal, real-world behavior, neither Consumer Reports’ nor Ford’s tests utilized styles and conditions at either end of the driving spectrum. Jake Fisher, CR’s director of automotive testing, explained to the Detroit Free Press that there is no evidence that Ford was fudging any data on purpose:

Ford may have designed its vehicles to score well on the EPA test and the results are not being duplicated in real life, Fisher said.

Consumer Reports also stated that the Ford hybrid Fusion and C-Max “still deliver excellent fuel economy” even if their mpg figures were adjusted to reflect the new findings. The problem is that even if the fuel economy is good, it doesn’t measure up to driver’s expectations. “Most buyers won’t get anything near 47 mpg in the real world,” says Fisher. “Even though these two Ford hybrids are very efficient, this big discrepancy may leave customers disappointed.”

The main issue here is trust. Ford seems to understand the importance of trust better than anyone. In its just-released list of 13 Trends for 2013, #1 on list is the concept that “Trust Is the New Black.” The Ford press release explained why trust is so important:

Trust as a differentiator: Given its relative scarcity, trust is emerging as a key positive differentiator for brands. Correlation of trust to brand equity has increased by 35 percent since 2009. As a result, brands are having to rethink how they communicate with and reveal themselves to consumers – the more real and authentic they are, the better